1. Field of the Invention
This invention relates to novel polyimides which exhibit good dimensional stability, low water absorption and high mechanical properties and are adapted for use as substrates such as finely patternized flexible printed board substrates. The invention also relates to processes for preparing such polyimides as mentioned above.
2. Description of the Prior Art
It is known in the art that polyimide resins have very good resistances to heat and chemicals and very good electric and mechanical properties as well as other good characteristics.
A typical polyimide is one, for example, set forth in Japanese Patent Publication No. 36-10999, which is obtained from 4,4'-diaminodiphenyl ether and pyromellitic dianhydride. This polyimide has flexing ether bonds in the main chain thereof and is thus flexible irrespective of the fact that it consists of a totally aromatic polyimide. However, the polyimide has a low modulus of elasticity, an undesirably high coefficient of linear expansion and is poor in dimensional stability.
In recent years, there is a demand for the development of polyimide resins which have good dimensional stability along with good mechanical strength in fields such as finely patternized flexible printed boards. Currently employed polyimide resins exhibit a coefficient of linear expansion as undesirably high as about 3.times.10.sup.-5 /.degree.C., with the attendant problem that when laminated with metal sheets, are apt to warp or curl.
Moreover, there is an increasing demand for polyimide resins which have better thermal dimensional stability and good mechanical strength. Accordingly, extensive studies have been made on the improvement in properties of polyimide resins. To this end, many attempts have been made to use two or more aromatic diamines to improve the mechanical strength and thermal dimensional stability. However, in any of these attempts, it has not been possible to satisfy both requirements for thermal dimensional stability and good mechanical characteristics of polyimide resins.